Process for the production of a battery separator

ABSTRACT

In an alkaline dry cell battery separator being a laid mat of non-dissolvable polyvinyl alcohol fibers held together by a matrix of dissolved or partially dissolved dissolvable polyvinyl alcohol fibers, the improvement comprising the mat having up to 85% of cellulosic fibers relatively uniformly distributed in and among the non-dissolvable polyvinyl alcohol fibers and held therein by said matrix.

This is a divisional application of Ser. No. 136,334, filed Dec. 22,1987, now U.S. Pat. No. 4,767,687.

The invention relates to battery separators, and more particularly toseparators for use in alkaline cell batteries. Even more particularly,it relates to such separators which are made of polyvinyl alcoholfibers.

BACKGROUND OF THE INVENTION

Battery separators are distinguished in the art as primary batteryseparators and secondary battery separators. A secondary batteryseparator, such as a separator for a conventional lead/acid battery,requires very different properties as opposed to a separator for aprimary battery, e.g., an alkaline cell battery. Secondary batteries,normally, are rechargeable many times, while primary batteries, ifrechargeable at all, are rechargeable to a very limited degree. As aresult, the materials required for a secondary battery separator aresubstantially different from the materials required for a primarybattery separator.

A number of materials have been used in the prior art in connection withsecondary batteries, but the acceptability of those materials forprimary batteries cannot be predicted from acceptability in a secondarybattery, and, most often, separators useful in a secondary battery arenot useful in a primary battery. While a wide range of separators havebeen successfully used in secondary batteries, e.g., plastics, wood,wood pulp, rubber, and the like, materials which have been foundacceptable for primary batteries are far more limited. This isparticularly true in regard to alkaline primary batteries, such as aconventional alkaline cell battery, since the mode of manufacturethereof is considerably different from the mode of manufacture of aconventional secondary battery, e.g., a lead/acid automobile battery,and an alkaline cell primary battery separator must be capable ofoperating in a highly alkaline medium, as opposed to a low pH acidmedium.

Thus, acceptable separators for primary batteries, and especiallyalkaline cell batteries, have been substantially limited in the art.However, the preferred alkaline cell battery separator is made ofpolyvinyl alcohol fibers. These fibers are, essentially, unique in thisart, in that they can be formed into a flexible mat of thincross-sections to allow usual manufacture of the cells, but at the sametimes these fibers are stable, particularly dimensionally stable, at thehigh alkaline pHs of an alkaline cell battery. Accordingly, most modernalkaline cell batteries use battery separators made of polyvinyl alcoholfibers.

In order to form a mat of the polyvinyl alcohol fibers, dissolvable orpartically dissolvable polyvinyl alcohol fibers are mixed withnon-dissolvable polyvinyl alcohol fibers in a convenient solvent,usually water. After sufficient dissolution of the dissolvable polyvinylalcohol fibers, the mixture is then formed into a shape-sustaining form,e.g., a mat, and dried. The dissolved polyvinyl alcohol forms a matrixabout the non-dissolvable polyvinyl alcohol fibers and thus keeps thatmat in a shape-sustaining form. In addition, that matrix forms apermeable barrier between the polyvinyl alcohol fibers for appropriateionic transfer during discharge of the alkaline cell battery.

While such battery separators are the preferred form in the art, they dosuffer from several disadvantages. Firstly, due to variables inmanufacture, especially the above-noted dissolving step, the strength ofthe battery separator itself may vary considerably. The matrix formed bythe dissolved polyvinyl alcohol is not a particularly strong matrix, andthe polyvinyl alcohol fibers, themselves, are not particularly strongfibers in the wet state. Thus, in the manufacture of the batteryseparators, differences in the non-dissolved fibers and in the dissolvedmatrix can result in a formed mat that is subject to tearing. Further,the polyvinyl alcohol matrix, while providing a permeable matrix, tendsto produce considerable variation of permeability, which results inuneven ionic transport across the battery separator, and somewhatvariable electrical discharge thereof. Also, the process ofmanufacturing both the non-dissolvable polyvinyl alcohol fibers and thedissolvable polyvinyl alcohol fibers inherently produces variabilitiesin these fibers. This results in variability in the matrix formed by thedissolved fibers and in the permeability property, of thenon-dissolvable fibers/matrix forming the battery separator.

However, probably of more importance than any of the foregoingdisadvantages of these conventional alkaline cell battery separators isthe penetration of those conventional battery separators by dendriteformation. As is well known, in such alkaline cell batteries, themetallic component of the battery, e.g., lead or zinc, is separated fromthe other battery components by the battery separator. That batteryseparator, during manufacture of the battery, is wetted with a highlyalkaline solution in order to provide an ionic transport between the twocomponents of the battery. During use of the battery, and even duringnon-use and during storage, dendritic structures form from the metalcomponent of the battery. If these dendritic structures continue to formand enlarge, they can pierce through the battery separator and contactthe other component of the battery, thus, providing a direct short ofthe battery, and, of course, resulting in a decreased life orunserviceability of the battery. While the polyvinyl alcohol batteryseparators are resistant to the highly alkaline solution wettedtherewith, these conventional separators are relatively easily piercedby such dendritic structures, and the variability in porosity of theseparators allows more easy formation of the dendritic structuresthrough the battery separator, causing such shorts in the battery.

In view of the foregoing, the art has long sought methods of improvingthe relatively unique polyvinyl alcohol battery separator. However, forthe reasons explained above, this effort in the art has been difficultand unsuccessful, primarily because of the demanding properties ofalkaline cell battery separators. It would, therefore, be of substantialadvantage to the art to provide improved polyvinyl alcohol alkaline cellbattery separators, which mitigate the disadvantages noted above.

BRIEF DESCRIPTION OF THE INVENTION

The invention is based on several primary discoveries and severalsubsidiary discoveries. As a first primary discovery, it was found thatthe variabilities in permeability, of the polyvinyl alcohol separatorcould be substantially improved if the separator contained anappreciable portion of other fibers which have less variability, on andas a manufactured basis, than the variability of the polyvinyl alcoholfibers. This, accordingly, produces a more homogeneous separator and onewith substantially more predictability of its properties, and especiallypermeability.

As a subsidiary discovery in this regard, it was found that up to about85% of such other fibers could be added to the polyvinyl alcohol batteryseparator and still retain the essential properties of the polyvinylalcohol separator, when the other fibers are of very specialcharacteristics. With such large amounts of other fibers added to theseparator, substantial increases in the predictability of properties,noted above, and a substantial mitigation of the disadvantages of thepolyvinyl separator could be achieved. As a further subsidiarydiscovery, it was found that with as little as 5% of such other fibers,substantial improvement in the separator could be achieved.

As a second primary discovery, it was found that the other fibers, inorder to be effective in the battery separator for the reasons notedabove and to be useful in improving the battery separator, must be acellulosic fiber, as opposed to the many other fibers available to theart. As a subsidiary discovery in this regard, it was found that thecellulosic fibers should be either a natural cellulosic fiber, or a woodor plant pulp cellulosic fiber, and especially such fibers of certaincharacteristics.

As a third primary discovery, it was found that the dissolved polyvinylalcohol well adheres to the cellulosic fibers and incorporates thosefibers into the separator in a permanent manner. It was also found thatthe cellulosic fibers very substantially improved the predictability ofpermeability in the separators when used in higher amounts. As asubsidiary discovery in this regard, it was found that the cellulosicfibers could be placed into the battery separator simply by addition tothe conventional process for producing polyvinyl alcohol batteryseparators. This is a very substantial advantage to the presentinvention.

However, as important as the above discoveries are, a further veryunexpected and surprising discovery was made, which is the basis of amajor feature of the present invention. In this regard, in testing theimproved battery separators of the nature described above, it was quiteunexpectedly discovered that these improved battery separators verysubstantially decreased the dendritic formation in the batteries madewith the separator. While the explanation of the very unexpected andsurprising property is set forth more fully below, briefly, much moreuniform permeability (porosity) of the present separator, withattendantly much smaller pore size, interferes with growth of dendritesand therefore largely avoids sufficient formation of the dendrites thatthe dendrites can pierce the battery separator and cause a shorting ofthe battery.

Thus, briefly stated, the present invention provides an alkaline cellbattery separator which is a laid mat of non-dissolvable polyvinylalcohol fibers held together by a matrix of dissolved, or partiallydissolved, dissolvable polyvinyl alcohol fibers, and wherein theimprovement comprises the mat having up to 85% of cellulosic fibersrelatively uniformly distributed in and among the non-dissolvablepolyvinyl alcohol fibers and held therein by the said matrix.

Similarly, the present invention provides an alkaline battery whereinthe improvement is that battery having as the battery separator the laidmat described above.

The invention also provides a process wherein such a separator isprepared by mixing the non-dissolvable polyvinyl alcohol fibers, thedissolvable polyvinyl alcohol fibers, and the cellulosic fibers in asolvent liquid until the dissolvable fibers at least partiallydissolved. A mat is formed from the fiber mixture and then dried to ashape-sustaining form.

Other important features of the invention will be described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a preferred form of the process of theinvention; and

FIG. 2 is a highly idealized diagrammatic illustration of thearrangement of the non-dissolvable polyvinyl alcohol fibers, thecellulosic fibers, and the dissolved polyvinyl alcohol (forming a matrixfor the mat).

DETAIL DESCRIPTION OF THE INVENTION

Considering first, the process of the invention, as noted. above, amajor feature of the present invention is that the present process canbe carried out without any substantial complication of the conventionalprocess for producing polyvinyl alcohol (hereinafter PVA) batteryseparators. Thus, as shown in FIG. 1, the basic steps of that processare mixing dissolvable PVA fibers and non-dissolvable PVA fibers andcellulosic fibers so as to dissolve or partially dissolve thedissolvable PVA fibers in the solvent (usually water), The dissolvedPVA, in the solvent, permeates in and around the non-dissolved PVAfibers and cellulosic fibers, and upon forming a mat of thenon-dissolvable PVA fibers and cellulosic fibers, that solution of thePVA forms a matrix around the non-dissolvable PVA fibers and cellulosicfibers. That mat is then dried, and upon drying, the matrix of thedissolved PVA forms a shape-sustaining form of the mat.

In this latter regard, as highly diagrammatically illustrated in FIG. 2,the non-dissolvable PVA fibers 1 are intertwined among themselves, andthe dissolved PVA fibers forms a matrix 2 with spaced apart permeableportions thereof, and with discrete pores 3. Thus, the non-dissolvablePVA fibers are held in a shape-sustaining form only by the permeablematrix 2. Since the dissolvable PVA fibers are, as manufactured,somewhat variable, during any one processing of the battery separator,more or less of those fibers may be dissolved. Further, some of thenon-dissolvable fibers, due to manufacturing variabilities andprocessing conditions in forming the mat, may either dissolve orpartially dissolve. FIG. 2, however, shows the case where none of thenon-dissolvable fibers have partially dissolved and where all of thedissolvable PVA fibers have dissolved and the residues thereof formmatrix 2. This is, of course, the ideal situation but one which does notalways occur in normal manufacture. Thus, in other processing, aremainder of dissolvable, but undissolved PVA fibers may be present, andoften are, as well as some partially dissolved non-dissolvable fibers.When lesser amounts of the PVA are in the matrix, the matrix is ofgreater permeability and greater pore size and less strength, or visaversa.

On the other hand, since matrix 2 results from the drying of the residueof dissolved PVA, the actual disposition of that matrix may beconsiderably varied. Accordingly, the permeability and pore size anddistribution thereof in the separator will vary from place to place oreven from processing to processing.

However, by placing in the separator cellulosic fibers 4, a totallynon-dissolvable fiber is placed into the separator. While for claritypurposes, FIG. 2 only shows a small number of non-dissolvable fibers,cellulosic fibers, dispersed matrix and irregular pores, in actualpractice, these fibers, matrix and pores would form a substantial opaquemat. It can therefore be appreciated that the presence of the totallynon-dissolvable cellulosic fibers provides a far more predictablepermeability of the separator, since all of the cellulosic fibers placedin the mixture from which the mat is formed will reside, precisely, inthe formed mat. When higher amounts of the cellulosic fibers are placedin the separator, e.g., approaching about 85%, those amounts ofcellulosic fibers will substantially affect the predictability of thestrength and permeability of the separator.

The cellulosic fibers have a further advantage in that they are, bynature, relatively rough surfaced fibers, as shown in FIG. 2, as opposedto the surface texture of the PVA fibers. These relatively roughsurfaced cellulosic fibers provide increased strength of the intertwinedand intermatted PVA fibers and cellulosic fibers due to this roughsurface texture. In other words, the cellulosic fibers have greater"binding" ability than the PVA fibers.

All of the foregoing was developed during the course of research leadingto the present invention and the improvements sought by that research,as explained above, were realized. However, in testing of the improvedbatter separator in actual battery construction and use, a mostunexpected and surprising discovery was made. Those tests showed thatquite surprisingly, the dendritic formation in batteries constructed ofthe present improved battery separator was substantially suppressed.Further studies into the suppression of dendritic formation by thepresent improved battery separators have shown that such dendriticformation suppression is a result of the physical configuration of thepresent battery separator. This can be understood by a comparison of theconventional PVA battery separator, opposite the present improvedbattery separator containing the cellulosic fibers. As discussed above,the PVA fibers form unpredictable areas of matrix formation andunpredictable distribution of pores and pore sizes. Some portions of thebattery separator, for example, may have larger pore sizes, while othershave smaller pore sizes, but the average pore size is acceptable forionic transfer. However, the existence of the larger pore sizesfacilitates dendrite formation through those larger pores. Once dendriteformation pierces through the separator, and a short occurs, the batterywill commence losing its energy. As more and more dendrite formationspierce the battery separator the battery will commence draining at amore rapid rate until the battery becomes weakened or evenunserviceable.

As opposed to this, however, cellulosic fibers may be very uniformlydistributed in forming the PVA battery separator. These cellulosicfibers bridge through and around pores and in that manner effectivelyregulate the pore size formed in the PVA separator. Thus, while the PVAseparator may have large pores formed by the matrix of the PVA, in thoselarge pores, many cellulosic fibers will bridge and effectively createsmaller pores than the pore created in the formation of the PVA matrix.The smaller the pore, of course, the more difficult for the dendrites toform in that pore and pierce the battery separator. Thus, the larger theamount of the cellulosic fibers, the more uniform and smaller is thepore size, and the greater the reduction in dendrite formation which canpierce the battery separator. As the amount of cellulosic fiberssubstantially increases in the battery separator, this effect becomesmore pronounced and the formation of dendrites is correspondinglysubstantially decreased. Further, it was found that as even morecellulosic fiber is used in the PVA separator, the fibers form anindepth tortuous porosity. Dendrite formation is essentiallyperpendicular to the metal from which the dendrites form and thetortuous path through the indepth pores of the cellulosic fibersinterferes with this perpendicular formation of the dendrites. Thusdendrite formation to the extent that the dendrites can pierce theseparator is largely avoided with higher amounts of cellulosic fibers.

With this discovery, additional testing showed that the size of thecellulosic fibers also affected dendrite formation which could piercethe battery separator. As the fibers are smaller and smaller in size,the fibers form smaller pores and more tortuous pores indepth, which, asexplained above, further suppresses dendrite formation and penetrationof the battery separator. Thus, it was discovered that even furtherimproved battery separators could be provided when the cellulosic fibersare of very small size. In this regard, the term size refers both tofiber diameter and fiber length, since it will be appreciated that bothdiameter and length affect the ability of the cellulosic fibers toevenly distribute in the PVA battery separator and to achieve thesmaller effective pore size and the tortuous indepth pore size. As morefully explained below, such characteristics of the cellulosic fiber areeasily obtained.

Turning again to FIG. 1, the process of the present invention can bepracticed with the usual equipment for producing PVA battery separators,i.e., conventional paper making machines, and no changes in theapparatus are required, with the one preferred exception discussedbelow. This is a substantial advantage of the present invention, in thatthe superior separator of the present invention can be produced withoutany substantial changes in the processing equipment. However, theprocess itself must be slightly altered.

Thus, instead of introducing only the dissolvable PVA fibers and thenon-dissolvable PVA fibers into the mixing apparatus, in addition, thecellulosic fibers must also be introduced. It is necessary, as can beappreciated from the above, that the cellulosic fibers be relativelyuniformly dispersed throughout the mixture. Otherwise, the interminglingof the fibers, in a uniform disposition, to provide increased uniformityof strength and pore size will not be provided. However, the cellulosicfibers are relatively strong fibers and can withstand considerablemixing without substantial deterioration. The non-dissolvable PVAfibers, likewise, can stand considerable mixing, and for these reasonssufficient mixing can be and is accomplished to ensure a relativelyuniform disposition of the cellulosic fibers in the mixture.

Once this mixture is formed, a mat is produced in the conventionalmanner on a conventional paper making machine, e.g., placing the mixtureon a wire or screen to form a mat thereof. Likewise, the mat is dried toshape-sustaining form in a conventional manner, e.g., by de-watering anddrying on cans, with or without vacuum. All of this is quiteconventional in the art and need not be described herein in any furtherdetail. However, generally speaking, the drying step should be carriedout at a temperature which will not substantially degrade the PVAfibers. Conventional drying temperatures of about 50° to 270° F., of themat more preferably about 80° to 150° F., are therefore preferred. Thisis especially true when the solvent used in the mixing step is water,which is the preferred solvent.

Thc cellulosic fibers are essentially imperious to the water solvent,e.g., will not substantially swell, degrade or the like during themixing process. This is true even at higher water temperatures tofacilitate the dissolution of the dissolvable PVA fibers. Thedissolvable fibers dissolve at different rates and to different extendsdepending upon the temperature of the water and the degree of mixing. Inthe conventional processes, the temperature of the water solvent isusually from about 50° F. up to about 100° F., and such temperatures canbe used and are preferred with the present process. However, thetemperature of the mixing step can be as great as 150° F., but morepreferably no greater than 120° F., e.g., between 50° and 120° F.

On the other hand, there must be sufficient mixing, at an appropriatetemperature, such as the dissolvable fibers dissolve sufficiently thatupon drying the dissolved PVA produces a shape-sustaining matrix for theremaining fibers, and the matrix is sufficient to allow cutting andshaping of the mat. Accordingly, the temperature of the water, thedegree of mixing, the specific solubility of the dissolvable fibers, andthe specific solubility of the non-dissolvable fibers must all be takeninto account to assure this result, as well as the result of uniformdispersion of the cellulosic fibers in and among the non-dissolvable PVAfibers. Some experimentation, therefore, may be required in this regard,but that experimentation can be easily accomplished with only a limitednumber of test runs.

It will also be appreciated that there must be sufficient dissolvablefibers in the mixture to assure an adequate matrix of the residue of thedissolved PVA. While this will vary widely, depending upon theparticular dissolvable fibers and non-dissolvable fibers, as well as theamount of cellulosic fibers used, generally speaking, the ratio of thenon-dissolvable fibers to the dissolvable fibers is from about 9:1 to1.5:1, but more preferably about 3.5:1.

As will also be appreciated, in order to achieve an uniform mixture ofthe non-dissolvable fibers and the cellulosic fibers, the fiber geometryof each is of importance. The non-dissolvable fiber should not have anaverage denier greater than about 10, or otherwise the fiber will berelatively stiff and difficult to uniformly disperse. On the other hand,the average denier should not be so small that the fiber could bedamaged in the mixing. Therefore, the preferred average denier of thenon-dissolvable fiber is from about 0.5 to 3 and more preferably fromabout 0.5 to 1.

Likewise, the average length of the fibers will affect the ability touniformly disperse the fibers among themselves. Thus, it is preferredthat the non-dissolvable fiber have an average length of no greater thanabout 12 mm. On the other hand, the fiber should be sufficiently long toachieve substantial intertwining of the fibers, and to this end theaverage fiber length should be at least about 3 mm. More preferably, thefiber will be about 3 to 5 mm.

While as noted above, up to about 85% of cellulosic fibers can be usedin forming the separator, and at these higher amounts the improvementsdescribed above will be more pronounced, at this level of cellulosicfibers, the properties of the separator begin to change from that of aseparator made of PVA fibers to a separator made of cellulosic fibers.The properties of a separator made of all cellulosic fibers are notsufficient for separator application, because the overallcharacteristics become unacceptable. Therefore, while up to 85%cellulosic fibers may be used, it is preferred that no more than about60% or 50% be used. However good results are achieved when only 30% ofcellulosic fibers are used. On the other hand, while only a few percentof cellulosic fibers will provide some improved properties, forsubstantial improved properties at least 5% of the cellulosic fibersshould be used, and more preferably at least about 10%. Ideally, theamount of cellulosic fibers will be at least about 10% and up to 40%.

As noted above, the cellulosic fibers are relatively critical in termsof the specific characteristics thereof. While the fibers may be anynatural cellulosic fibers or wood or plant pulp fibers, all of thesefibers can vary considerably in terms of the specific compositionsthereof, and especially in terms of compositions associated with thefibers. For example, if wood pulp fibers are used, these fibers must beremoved from associated compositions, such as lignin, hemicellulose, andthe like in order to provide the relatively pure cellulosic fibers.Otherwise, substantial amounts of these associated compositions cancreate substantial difficulties in the battery separators. Plant pulpfibers can have similar associated compositions, but in addition mayhave other compositions such as sugars and the like. Here again, whenthese fibers are used, the fibers should be separated from theassociated compositions such that relatively pure pulp fibers are used.

A preferred form of the cellulosic fibers is that of cotton fibers,which are natural cellulosic fibers of relatively high purity. A pureform of cotton fibers is cotton linter fibers. While historically,cotton linter fiber were recovered from textile processing, as arelatively pure form of short, staple length cotton fibers, and suchfibers are still available, that term is now used in the art to includecellulosic fibers of essentially the same purity as cotton linterfibers, but derived from other plant sources. For example, wood andplant pulp fibers may be treated so as to essentially remove the lignin,hemicellulose, sugars, and the like, and those recovered fibers are alsoreferred to in the art as cotton linter fibers. Thus, the term "cottonlinter fibers" is intended herein to mean not only the natural occurringcotton linter fibers but the manufactured forms thereof from other plantmaterials.

Cotton linter fibers are preferred, since, as noted above, the lintersare relatively pure, short, staple length fibers, and these short,staple length fibers are able to provide the increased uniformity ofpore size and the tortuous porosity described above. However, the sameeffect can be achieved by mechanically shortening other naturalcellulose fibers. For example, natural cotton fibers can be mechanicallyshortened by a variety of textile treating machinery so as to producefrom the relatively long natural cotton fibers very short staplelengths. Apparatus to achieve this effect is known as a refiner, and adouble disk refiner is a typical example thereof. In a double diskrefiner, a conical member having teeth, is rotated in an outer conicalshell and the cotton is passed therebetween. Cotton fibers so processedwill have average staple lengths considerably shorter than the naturalcotton fiber and the average staple length actually achieved in suchrefiner is dependent upon the clearance between the conical sections,the design of the teeth and the like. With such apparatus, well known tothe art, natural cotton fibers can be reduced in average fiber length tovery short fiber lengths, even shorter than cotton linter fibers.Therefore, if natural cotton fibers are used, as opposed to cottonlinter fibers, then it is greatly preferred that the natural cottonfibers be mechanically shortened in staple length, although naturalcotton fibers without shortened staple lengths may be used in producingthe battery separators, but in this case the improvements discussedabove are somewhat lessened. There is no theoretical lower limit on thelength of the cotton fibers, either mechanically shortened or cottonlinter fibers, since the shorter the fibers the more pronounced are theimprovements discussed above. However, from a practical and processingpoint of view, the cotton fibers, along with the PVA fibers, must beformed into a mat. Thus, the cotton fibers cannot be so short thaL theydo not mat in conventional machines. If the fiber lengths are too short,the fibers will be removed from the mat during the usual mat formationand will not be retained in the mat. Thus, so long as the cotton fiberswill be retained in the mat formation and not pulled through the formingscreen, those lengths are acceptable for the present purposes. Indeed,the shorter the cotton fibers the better for purposes of the invention,so long as the fibers will be retained on the forming screen. On theother hand, since the longer the average staple length, the lesspronounced the advantageous of the invention, it is also preferred thatwhatever the source of the cellulosic fibers, that the fibers have anaverage denier and staple length no greater than that of natural cottonfibers, and especially less than that of natural cotton fibers, e.g.,less than one-half or one-fourth the staple length of natural cottonfibers. Thus, if manufactured cotton linter fibers or other cellulosicfibers are used, those fibers, as manufactured, or as reduced in lengththrough a refiner or the like, should have an average staple length asdescribed above.

Especially at the high levls of cotton fibers, i.e., up to about 85%, achange in the usual process is preferred. As can be appreciated, cottonfibers, or other cellulosic fibers of similar nature as discussed above,are subject to shrinking. This is particularly true in view of theprocessing required to form the PVA battery separator, especially thehigher temperatures and in the wet condition. With greater amounts ofthe cellulosic fibers, the amount of shrinkage of the mat can beappreciable. While the shrinkage does not adversely affect theperformance of the battery separator, it does complicate the manufacturethereof. In this regard, it is important to have predictable widths ofthe manufactured mat in order that those widths may be appropriately cutto sizes for battery separators without such cutting resulting in undueamounts of scrap (portions left over after cutting to appropriate sizesfor battery separators). To substantially reduce such shrinkage, thecellulosic fibers, and especially natural cotton fibers, can be easilymercerized in a conventional mercerization process and such willsubstantially reduce the shrinkage of the resulting mat. The details ofmercerization are well known to the art and need not be recited hereinfor sake of conciseness. The usual mercerization conditions are quiteacceptable for purposes of the present process. Indeed, mercerizedcotton and cotton linter fibers are available in the market.

In regard to the battery separator itself, in order to better functionin an alkaline cell, the so produced mat preferably has a dry weight ofabout 0.6 to 1.4 1bs per 100 square feet, and especially about 1.2 lbsper 100 square feet. Thus, the process should be conducted so that theconcentration of the fibers in the mixture, along with the rate ofdischarge thereof for matting and drying purposes, are such that mats ofthose weights will be produced. This, however, is the conventionalmethod of producing such battery-separators, and the particularconcentration of fibers in the mixture can be according to theconventional concentrations, e.g., about 0.1% to 5%, more preferablyabout 0.4% to 1.0%. Likewise, the conventional paper making apparatusused for producing the battery separators can be operated in theconventional manners for producing mats of these weights.

The use of cellulosic fibers provides yet a further advantage to thepresent invention. Especially with higher amounts of cellulosic fibers,the formed battery separator mat is susceptible to calendaring. Bycalendaring, the formed mat can not only be densified, which againserves to decrease the average pore size for the reasons explainedabove, but also provides the battery separator with a very predictablethickness. The predictability of the thickness of the battery separatoris important in the manufacture of the alkali cell battery. For example,in a typical battery manufacture, the clearance between the two batterycomponents, which is occupied by the separator, may be 13 mils. When thebattery separator is placed in that space to separate the two batterycomponents and wetted with alkali solutions, the battery separator willexpand with the impregnation by the alkali solution. If the formed matof the battery separator is not of predictable thicknesses, placing ofthe separator in the battery and the expansion thereof upon being wettedby the alkaline solution can cause the separator to either not snugglyfit in the space provided at all portions of the battery or be undulycompressed by those portions. With the present battery separator, themat can be calendared to a predictable thickness, so that it can bereadily manufactured in the battery (in the space provided) andtherefore will predictably swell to the correct thickness. For example,the present separator material can be calendared on conventional heatedcan calendars with one or more nips at temperatures between 150° and250° F., more usually between 180° and 200° F., with nip pressures from100 to 500 pounds per linear inch, especially between about 300 and 400pounds per linear inch. With such calendaring, for example, a matformation of 13 mils can be densified and reduced in thickness to, forexample, 4 mils or so, or 6 mils or 8 mils and allow for easymanufacture and predictable swelling when wetted with alkalie solution.While calendaring is not necessary for purposes of the invention, thedensification during calendaring does provide improved advantages, asdiscussed above, and provides the special advantage of a verypredictable manufacture of the battery. This is particularly true withuse of higher amounts of cellulosic fibers, e.g., amounts of 30% orgreater.

It will thus be seen that the invention provides a substantialimprovement in the art in that the present battery separators havegreater predictability of strengths, pore size, and permeability, lesssubject to unpredictable tearing in manufacture and fabrication intobatteries, while at the same time providing the very important reductionin dendrite formation and even the very predictable thickness of theseparator. The separators can be produced essentially, in the usualprocess, with no substantial changes in the apparatus and very littlechange in the process, which is important to the invention.

The invention will now be illustrated by the following Example, but itis to be understood that the invention is not limited thereto, butextends to the scope of the foregoing specification and the followingclaims. In the Example, as well as in the specification and claims, allpercentages and parts are by weight, unless otherwise indicated.

EXAMPLE

Into a conventional hydro pulper was placed 30 lbs. of cotton linters(HS225--98% pure alpha cellulose--Alpha Cellulose Corporation) and about850 gals. of water at room temperature to provide about 0.4%concentration of the cotton linter in the water. The hydro pulp wasoperated for about 30 minutes to well disperse the cotton linters. Thismixture was transferred to a conventional dump chest and to this mixturewas added, with high agitation, 53 lbs of non-dissolvable PVA fiber(Kuralon VPB 103, 1 denier,--average stable length of 3 mm-KurrarayCorporation), 17 lbs. of dissolvable PVA fibers (Kuralon VPB 105.2--1denier average--staple length 4 mm--Kurraray Corporation) and about 2000gals. of water at room temperature to provide about a 0 4% concentrationof the total mixture. This mixture was pumped to a conventional mix boxinto which was metered a 5% solution in water of a wetting agent (TritonX 114). The rate of flow of the wetting agent was adjusted to achieve awetting characteristic of the finished product as described below.

The mixture from the mix box was passed to a conventional wire screenpaper making machine and the speed of the machine was adjusted toproduce a dry weight of the subsequently formed mat of about 1.23lbs./100 square ft. The de-watering step formed a wet mat of the fibersand the wet mat was dried on conventional cans heated to about 270° F.to a moisture content of less than 1%.

An industry standard wet-out test was performed. A1 square inch sampleof the dried mat was held edge-wise over a cup of 35% KOH and droppedthereinto so that the sample floated on the KOH. The time required forthe KOH to wet through the sample was noted. The sample should wetthrough in less than 5 seconds but more than instantaneously. The flowrate of the wetting agent, discussed above, was adjusted until thewet-out time was approximately 3 seconds. The wet-out time wasperiodically measured during the course of each run and the flow rate ofthe wetting agent was adjusted accordingly.

The recovered dried mat had a weight of about 1.2 lbs/100 square foot, athickness of about 13 to 14 mils, a machine direction tensile strengthof about 40 lbs./inch, a cross-direction tensile strength of about 25lbs./inch, a burst strength of between 15 and 30 lbs./inch², a wet-outtime in 35% KOH of about 3 seconds, and a permeability (air) of about 40ft.³ /min./ft/². The product had no dimensional change in 35% KOH.

Part of the product was calendared with a nip pressure of 350 pounds perlinear inch to reduce the thickness to about 4.25 mils. Ihe properties,described above, essentially remained, but this reduced thickness mat ismost useful in high efficiency/low volume alkaline dry cell batteries.

What is claimed is:
 1. A process for the production of an alkaline drycell battery separator being a laid mat of non-dissolvable polyvinylalcohol fibers and cellulosic fibers held together by a matrix ofdissolved dissolvable polyvinyl alcohol fibers comprising:(a) mixing thesaid non-dissolvable polyvinyl alcohol fibers, dissolvable polyvinylalcohol fibers and cellulosic fibers in a solvent liquid until arelatively uniform mixture of the cellulosic fibers in the saidnon-dissolvable fibers is achieved and the said dissolvable fibers aredissolved; (b) forming a mat of the said fibers in the mixture; and (c)drying the mat to a shape-sustaining form.
 2. The process of claim 1,wherein the solvent is water.
 3. The process of claim 2, wherein thetemperature of the water of the mixture is between 50° and 120° F. 4.The process of claim 1, wherein the said mixing is such that upon dryingthe mat the dissolved polyvinyl alcohol produces a shape-sustainingmatrix for the remaining fibers and the matrix is sufficient to allowcutting and shaping of the mat.
 5. The process of claim 1, wherein theratio of the said non-dissolvable fiber to said dissolvable fibers isfrom about 9:1 to 1.5:1.
 6. The process of claim 5, wherein the saidratio is about 3.5:1.
 7. The process of claim 1, wherein thenon-dissolvable fibers have an average denier of about 0.5 to
 3. 8. Theprocess of claim 7 wherein the said denier is from about 0.5 to
 1. 9.The process of claim 1, wherein the non-dissolvable fibers have anaverage length of at least 3 mm.
 10. The process of claim 1, wherein thesaid mixture contains up to about 60% of cellulosic fibers.
 11. Theprocess of claim 10, wherein the said mixture has at least 10% and up to40% of cellulosic fibers therein.
 12. The process of claim 1, whereinthe cellulosic fibers have an average denier and staple length nogreater than that of natural cotton fibers.
 13. The process of claim 1,wherein the cellulosic fibers have an average staple length no greaterthan one-half that of natural cotton fibers.
 14. The process of claim 1,wherein the cellulosic fibers are natural cellulosic fibers or wood orplant pulp fibers.
 15. The process of claim 14, wherein the cellulosicfibers are cotton linter fibers.